Significant research has been conducted in recent years on the use of biological control agents to increase agricultural productivity and efficiency. These studies have shown that various microorganisms are able to suppress plant pathogens or supplement plant growth, thus offering an attractive alternative to chemical pesticides which are less favored because of their effect on human health and environmental quality.
Several screening programs have been used to isolate biological control agents effective in facilitating plant growth or combating pests in the laboratory or in the field. An example of one such biological control agent is Bacillus thuringiensis, which has been shown to produce toxic proteins lethal to certain insects. Another example is the bacterial strain Bacillus cereus UW85 (ATCC No. 53522), which has been found to protect alfalfa seedlings from damping off caused by Phytophthora medicaginis, tobacco seedlings from Phytophthora nicotianae, cucumber fruits from rot caused by Phthium aphanidermatum, and peanuts from Sclerotinia minor (See U.S. Pat. No. 4,877,738). In addition, Bacillus cereus AS4-12 (ATCC No. 55609) has been found to increase the efficacy in fostering the growth and establishment of alfalfa plants in the upper mid-western United States (See U.S. Pat. No. 5,552,138).
Earlier experiments have shown that strains of Pantoea agglomerans may also be able to increase the yield of legumes and inhibit the growth of phytopathogenic fungi. Hoflich and Ruppel, “Growth stimulation of pea after inoculation with associative bacteria,” Microbiol. Res., 149:99-104 (1994). P. agglomerans (formerly Enterobacter agglomerans) is a Gram-negative Enterobacterium often found associated with plants, water, soil, or foodstuffs. P. agglomerans is also a diazotroph, and able to fix nitrogen in both pure culture and in association with wheat. Merbach et al., “Dinitrogen fixation of microbe-plant associations as affected by nitrate and ammonium supply,” Isotopes Environ. Health Stud., 34:67-73 (1998). It has also been reported to produce two auxins and two cytokines in pure culture. Scholz et al., “Development of DAS-ELISA for some selected bacteria from the rhizosphere,” Zentralbl. Mikrobiol. 146:197-207 (1991); Scholz-Seidel C. and Ruppel S., Nitrogenase and phytohormone activities of Pantoea agglomerans in Culture and their reflection in combination with wheat plants,” Zentralbl. Mikrobiol. 147:319-328 (1992). Even with these studies, however, little is known about the interaction between P. agglomerans and cereal grasses, and whether P. agglomerans may serve as an effective biocontrol agent.
Klebsiella pneumoniae is also a member of the family Enterobacteriaceae and a known nitrogen fixing bacterium, i.e. able to convert atmospheric nitrogen into ammonium. K. pneumoniae is a free-living soil bacterium and unlike other nitrogen-fixing bacteria, such as Rhizobium, K. pneumoniae does not participate in symbiotic interactions with leguminous plants. K. pneumoniae has also not yet been shown to be effective in enhancing the growth of cereal grasses.
The mechanisms by which biological control agents are able to increase agricultural productivity and efficiency are diverse, and will vary depending upon the unique characteristics of each particular agent. It is believed, for example, that certain bacteria are able to control root rot in plants by competing with fungi for space on the surface of the plant root. It is also believed that competition between various bacterial strains in a plant's native microflora may stimulate root growth and increase the uptake of mineral nutrients and water to enhance plant yield. Alternatively, toxins produced by certain bacterial species are believed to facilitate plant growth by controlling bacterial species pathogenic to the plant. Bacterially produced antibiotics are an example of such toxins.
Some have suggested that bacterial strains other than those presently identified may also prove to be beneficial to crop plants. In particular, it is quite possible that some of these bacterial strains may be particularly helpful in cultivating various field crops as a result of relationships formed between plant and bacteria. The present invention discloses several such bacterial strains.